Production of diaryl compounds



Patented July 16, 1940 J UNITED STATES PATENT OFFICE PRODUCTION OF DIARYL COMPOUNDS can F. Prutton, Cleveland, Ohio, assignor to The Dow Chemical Company, Midland, Mich., a corporation of Michigan 7 No Drawing. ApplicationJuly 20, 1935, Serial No. 32,447

3 Claims. (01. 260-670) This invention relates to improvements in tinuous operation of the process. The invention is processes for producing diphenyl and higher based upon my discovery that a preliminary cardiaryl compounds by the pyrolysis of benzene and burizing treatment of the iron surfaces of the its homologues, or mixtures thereof. I pyrolyzing apparatus serves to suppress the 5 One of the difficulties commonly associated catalytic actionof iron in causing formation of '5 with processes in which benzene hydrocarbons carbon in the apparatus subsequently when benare heated at high temperatures is the formazene hydrocarbons are exposed to a pyrolyzing tion of the carbon on the heating surfaces. Such temperature therein. carbon formation causes a loss of materials in It is among the objects of the invention to process, while the carbon deposits so formed provide a method of pyrolyzing benzene hydro 10 interfere with the transfer of heat and frequently carbons, wherein the formation of carbon is I cause interruptions of the process for removing greatly reduced, or even substantially prevented, carbon accumulations which obstruct the apso that a process employing the method can be paratus. carried on continuously for long periods of time In apparatus for pyrolyzing benzene hydrocarwithout interruptions caused by carbon accumu- 15 bons, it is desirable to employ iron or steel as lations. Another object is to enable the use of the material of construction wherever it can be wrought iron or ordinary carbon steel as strucused without detriment. It isknown, however, tural material in apparatus for carrying out the that in the pyrolysis of hydrocarbons at high aforementioned pyrolytic'reaction. Other obtemperatures, e. g., in the range of approximately jects and advantages will appear in the following 20 500 to 900 0., iron promotes the formation of description in which the invention is fully set carbon. Particularly, in connection with the forth and particularly pointed out in the claims. pyrolysis of benzene hydrocarbons for the forma- In connection with experiments with the pyation of diphenyl and related diaryl compounds rolysis of benzene vapors in the temperature it has been found that much trouble is enrange of 650 to 850 C., in which the vapors were :25 countered due to the formation of carbon when passed through heated tubes of wrought iron or the process is carried out in iron apparatus, as carbon steel, it was found that within a few reported in the literature (cf. Smith and Lewhours at most'the tubes would become practically cock, J. Chem. Soc. 101, 1453-1459; Zanetti and blocked with carbon. This result occurred irre- Egloff, J. Ind. Eng. Chem. 9, 350-356). The spective of'the. purity of the benzene, or of the 30 formation of carbon deposits seriously interferes rate of vapor flow, the carbon formation, of with the regular and continuous operation of the course, being the more rapid the higher the temmethod, so much sothat it has generally been 'perature within therange stated. The accumularegarded as impracticable to use apparatus of tion of carbon in the apparatus necessitated wrought iron or carbon steel,particular1y tubufrequent shutdowns for cleaning out the car- 35 lar apparatus, in a commercial process for pyhim and indicated the practical impossibility of rolyzing benzene in the usual temperature range using ordinary iron or steel tubes in commercial of about 650-850 C., because the apparatus apparatus for making diphenyl and diaryl comwouldrapidly become filled with carbon and frepounds by pyrolyzing benzene hydrocarbons, in-

40 quent shutdowns would be necessary for cleanasmuch as a commercial process would be re- 40,

ing out the carbon. Various proposals have been quired to operate continuously over considerable made for avoiding or overcoming the aforesaid periods of time. difficulty, among which it has been suggested I observed, however, that carbon was formed to provide special coatings or linings on ordinary less rapidly in tubes which had been previously steel apparatus which inhibit or reduce the used for pyrolyzing benzene until filled with car- 45 for at o o C o 0 o introduce along with bon and from which the carbon had been cleaned the vapors of the benzene hydrocarbon a forout. In particular, I found that if care was takeign substance which has the effect of preventen not to scrape the surfaces of the iron tube in mg or counteracting the formation of carbon. removing the carbon, simply blowing out the I have now found a much simpler method than loose carbon and lightly brushing the surfaces 50 those heretofore proposed, by which carbon f0rto remove adhering particles, even better remation in the pyrolysis of benzene and benzene sults were obtained in the reduction of carbon hydrocarbons, when carried out in apparatus of formation when the cleaned tubes were later used iron or carbon steel, can be greatly reduced, so again in the pyrolysis. Apparently a layer of that it does not interfere materia y With the coniron carbide had been formed on the exposed 5s iron surfaces, which, once produced, prevented or counteracted further catalytic action of the iron itself in causing the carbon formation. In order to demonstrate the effectiveness of a preliminarycarburizing of the iron tubes prior to using the same for pyrolyzing benzene, the following experiments were conducted:

Example 1 A 1-inch mild steel tube was carburized by heating to about 900 C. for 9 hours while packed with charcoal. The carburized tube was then placed in a furnace, and connected at one end with a source of supply for benzene vapors, and at the'other end with a condenser. The tube was heated and benzene vapors passed therethrough at a rate corresponding to the vaporization of 5 cubic centimeters per minute, the temperature inside the tube reaching a maximum of 3 approximately 750 C. The hot vapors were condensed, and the unreacted benzene in the condensate was again vaporized and recirculated through the apparatus. After 9 hours of continuous operation the testwas discontinued, although no obstruction to thepassage of benzene vapors had occurred. A small amount of loose carbon was found in the tube, amounting to 4.5

grams. The percentage yield of diphenyl in this run was 62 per cent. In a check run under the same conditions, except that an uncarburized tube otherwise similar'to the first one was used, the tube'was completely stopped with carbon at the end of 5 hours, and 41.2 grams of carbon were removed from the tube after the run was stopped, while the yield of diphenyl was only 33 per cent. 4

Example 2 In a similar experiment a 1-inch steel. tube was used, which had been case-hardened on the inner surface by heating with sodium cyanide at about 900 C. for about 9 hours. Benzene vapors were circulated through the tube'at the same rate as before for 9 hours, while heating the sameto a maximum temperature of "about 750 C. No stoppage'of-vapor flow occurred during the time. At the'end of the run a quantity of '7 .2 grams of carbonflwas removed from the tube'.

I'have found that similar tubes" which'had been thoroughly carburized on the inner surface can be used continuously for as long as 450 hours or more for pyrolyzing benzene at a temperature between 700 and 750 C. without formation of suficient carbon therein to cause obstruction to the passage of vapors therethrough, whereas without such preliminary carburizing treatment they would have been completely stopped with carbon in a few hours.

My invention thus makes possible the commercial production of diphenyl and related diaryl compounds, and mixtures thereof, by pyrolysis of benzene hydrocarbons carried out in tubular or equivalent apparatus made of iron or carbon steel. The pyrolyzing tube or reactor parts need a only be thoroughly carburized on the surfaces which are to be exposed to the hydrocarbon vapors at a pyrolyzing temperature. Such carburiza'tion may be done in any of the ways known to the art, to produce on the iron a surface layer consisting chiefly or substantially of iron carbide.

'Othr modes of applying the principle of my invention may be employed instead of those explained, change being made as regards the method herein disclosed, provided the step or steps stated by any of the following claims or the equivalent thereof be employed.

I therefore particularly point out and distinctly claim as my invention:

1. In a method of pyrolyzing benzene hydrocarbons, the improvement which consists in preventing or reducing the formation of carbon by heating the benzene hydrocarbon vapors at a diaryl-forming temperature in an apparatus of iron or carbon steel,the surfaces of which exposed to contact with the vapors have been pre- :which consists in passing the hydrocarbon vapors" at a temperature between 650 and 850 C. 'over heated surfaces of iron or carbon steel which have been subjected to a carburizing treatment for'aitime' sufiicient to form a carbide coating thereon,

' CARL F. PRUT'I'ON. 

